1. Field of the Invention
The present invention relates to a junction or compound type optical element with layers of organic high molecular material on the surface of a glass substrate and also relates to a method for manufacturing it.
2. Description of the Prior Art
Optical elements such as lenses have been formed into desired shapes from glass blocks by grinding and polishing them. Glass material is advantageous in that there are various types of glass material with various refractive indexes and Abbe numbers, and that the glass can be ground and polished to have desired sizes or dimensions with high precision. Although the grinding and polishing of glass enables easy formation of spherical or flat surfaces, it is difficult to form specific surface shapes such as an aspherical surface by grinding and polishing. Hence, such specific surface shapes had to be formed by another method using grinding machines capable of achieving a greater precision, with a result of extremely high cost.
It is also known to form a desired shape by molding synthetic resin materials. This method is advantageous in that, unlike the grinding of the glass materials noted above, it can provide optical elements of desired shapes on a mass production scale at low cost. Nevertheless, it has the following shortcomings: In the first place, it is difficult to obtain synthetic resin materials of desired refractive indexes and desired Abbe numbers as compared with the glass materials. Also, no colored material nor the ones with a low transparency can be used for optical elements such as lenses that are required to transmit light. Moreover, the synthetic resins contract when they are hardened to impair the precision of the size and shape and cause internal distortion which result in degradation in the optical performances. Also, different from the glass materials, most of the synthetic resins lack durability. Thus these problems have so far precluded the optical elements of the synthetic resins to be used for optical products for which a high precision in their shapes are required.
To cope with those problems, compound optical elements were proposed wherein layers of resin materials are formed on an optical substrate e.g., glass-lens (see U.S. Pat. No. 2,464,738 and Japanese laid-open patent publication No. Sho 52-25651). These proposed elements are made by a method wherein a synthetic resin material of semi-polymerized state is interposed or sandwiched between a mold of a desired shape and an optical element, the polymerization is allowed to proceed with and to be completed and then the mold is removed so that the shape of the mold is transferred to the resin material. This method affords a comparatively easy selection of the refraction index as desired by forming the optical substrate with the glass materials. Moreover, as the surface of the optical substrate may be in any shape such as a spherical or flat one that is easily formed by grinding, the optical element can be manufactured at a comparatively low cost.
It is desirable that the synthetic resins to be used in this technique have a short hardening time for the sake of mass production and are not likely to contract upon hardening for achieving high precision transfer or imprinting and for minimizing the optical distortion. Also, because they form the outer surface of optical elements, they must be unaffected by surrounding air, i.e. they must have a high durability against the changes in the temperature and humidity of the air.
Thermo-setting resins such as epoxy resins and photo polymerizable bonding agent such as NOA-60 and Photobond.sup.12 (Brand name) have so far been proposed as the resin material for use in these techniques. Although the thermo-setting resin has superior imprinting characteristics and causes less distortion because of their small contractibility upon hardening, they are not suitable for a mass scale production because they must be heated for a long time at a high temperature for hardening. The hardening time may be shortened if drastically heated at a higher temperature but it may only result in a greater contraction upon hardening.
Conversely, the photo polymerizable bonding agent may be suitable for a mass production because of its capability of hardening in a relatively short time whereas it has a greater contractibility upon hardening and is inferior in durability.
As described above, it is difficult to find optimum synthetic resins for the compound optical elements. Here, to minimize the influence from the contraction upon hardening, it is desirable that the shape of the optical substrate materials resemble that of the surface to be obtained as closely as possible for minimizing the thickness of the synthetic resin layers. A problem, however, exists with this technique in that the difficulty in obtaining a shape approximate to the shape to be attained may increase as the complexity of the shape of the surface to be obtained increases.